Anisotropy and magnetic field dependence of the planar copper NMR spin-lattice relaxation rate in the superconducting state ofYBa2Cu

Abstract

The authors report measurements of the planar 63 Cu nuclear spin-lattice relaxation rates 63 H / u / ( 63 W| r ) for the static field oriented along the a (c) axis in the superconducting state of YBa2Cu307. These measurements were made on a single sample as a function of temperature and magnetic field. The weak-field results are well explained by several recent calculations in terms of a BCS spin-singlet, orbital-d-wave pairing state. They also find that at low temperatures bi W\ c varies linearly with the applied magnetic field, suggesting that at high field it is dominated by fluxoid cores. PACS numbers: 74.70.Vy, 74.30.Gn, 76.60.Es, 76.60.Gv Nuclear spin-lattice relaxation time measurements in superconductors have provided information about the nature of the superconducting state [l]. They have been employed extensively in the study of Cu02-based hightemperature superconductors [2,3]. Recently we reported [4] that measurements below T v of the relaxation rates of the planar 63 Cu, ,7 0, and 89 Y nuclei in YBa 2 Cu 3 0 7 had puzzling properties, especially when viewed in the framework of the antiferromagnetic Fermi liquid (AFL) theory [5], which is so successful in describing the NMR properties of the normal state of high-temperature superconductors. In this Letter we report significant new experimental results, and compare them with recent calculations of Bulut and Scalapino [6], Lu [7], and Lu and Pines [8], which successfully account for the results. These results suggest that YBa2Cu3<I>7 is a superconductor of the BCS variety with spin-singlet pairing and nodes in the gap (as in t/-wave pairing).In the normal state the nuclear spin-lattice relaxation rates n W\ a are anisotropic [r/=63, 17, or 89 corresponding to the planar 63 Cu(2), l7 0(2,3), or 89 Y nuclei, where a=a or c specifies the orientation of the static applied field]. For the Cu(2) in the normal state, b3 W ]a / b3 W u . = 3.4-3.7 (depending on the sample) and is independent of temperature and magnetic field strength [4,9]. In our earlier paper we reported that below T c the ratio dropped substantially and was temperature dependent, a result which was hard to understand within the AFL framework [4]. We studied the ratio at 8.31 T and also, at 77 K, in a weak field (0.45 T). The latter experiment proved that the change in b3 W\J^3W\ c just below T c was not an artifact resulting from the presence of the static magnetic field. However, our data for b3 W\ c . measured at zero field [nuclear quadrupole resonance (NQR)] in one sample and at 8.31 T in another sample had slightly different temperature dependences, deviating from each other progressively as one went to lower temperatures. It was not clear whether this was a sample dependence or a field dependence.In our present work, we have measured relaxation rates in strong, intermediate, weak, and zero fields in a single sample. We find that as one lowers T through T c in weak fields, the anisotropy ratio b3 W\ a / b3 W\ c first drops (as we showed previously) [4] then rises above the normal-state v...

Quasiparticle lifetimes in adx2−y2superconductor

“…Furthermore, the parameter values which are required are consistent with what is known about the Fermi surface [19][20][21] and spin-fluctuations from other measurements [6,7]. The analysis is based, however, on the subtraction of a constant relaxation rate from both quantities, an approach which must be justified within microscopic theory; we present a brief discussion of these issues and the light shed by our analysis on the behavior of the low-energy impurity scattering amplitude.…”

Motions of the flux lines in high-T c superconductors from NMR linewidth, relaxation and spin echo decay

“…One of the first indications of a contribution to relaxation from vortices was obtained from the field dependence and from the orientation dependence of 63Cu R~ in oriented powders of YBCO and LaSCO [21,22]. It was tentatively concluded that in the low temperature range the contribution comes from relaxation of nuclei from within the vortex core and subsequent spin diffusion to nuclear spins outside the core.…”

Anisotropy of the antiferromagnetic spin correlations in the superconducting state of YBa2Cu3O7 and YBa2Cu4O8

“…However, less attention has been payed on the ratios of these rates [2][3][4][5][6][7][8][9] in the superconducting state. A difficulty arises since in the normal state, the spin-lattice relaxation is largely insensitive to the strength of the applied magnetic field [10,11], while the relaxation rate becomes field dependent in the superconducting state at low temperature, with 1c [5][6][7]12]. In order to draw any conclusions about magnetism in the superconducting state it is therefore very important to look for intrinsic effects which can only be obtained from experiments done in weak magnetic fields so as to minimise the flux line influence.…”